Lock Compatibility (Database Engine)

Lock compatibility controls whether multiple transactions can acquire locks on the same resource at the same time. If a resource is already locked by another transaction, a new lock request can be granted only if the mode of the requested lock is compatible with the mode of the existing lock. If the mode of the requested lock is not compatible with the existing lock, the transaction requesting the new lock waits for the existing lock to be released or for the lock timeout interval to expire. For example, no lock modes are compatible with exclusive locks. While an exclusive (X) lock is held, no other transaction can acquire a lock of any kind (shared, update, or exclusive) on that resource until the exclusive (X) lock is released. Alternatively, if a shared (S) lock has been applied to a resource, other transactions can also acquire a shared lock or an update (U) lock on that item even if the first transaction has not completed. However, other transactions cannot acquire an exclusive lock until the shared lock has been released.

Applies to: SQL Server 2008 R2 and higher versions.

The following table shows the compatibility of the most commonly encountered lock modes.

Existing granted mode

Requested mode

IS

S

U

IX

SIX

X

Intent shared (IS)

Yes

Yes

Yes

Yes

Yes

No

Shared (S)

Yes

Yes

Yes

No

No

No

Update (U)

Yes

Yes

No

No

No

No

Intent exclusive (IX)

Yes

No

No

Yes

No

No

Shared with intent exclusive (SIX)

Yes

No

No

No

No

No

Exclusive (X)

No

No

No

No

No

No

Note

An intent exclusive (IX) lock is compatible with an IX lock mode because IX means the intention is to update only some of the rows rather than all of them. Other transactions that attempt to read or update some of the rows are also permitted as long as they are not the same rows being updated by other transactions. Further, if two transactions attempt to update the same row, both transactions will be granted an IX lock at table and page level. However, one transaction will be granted an X lock at row level. The other transaction must wait until the row-level lock is removed.